Acoustic touch position sensors are known to include a touch plate and two or more transducers each of which imparts a surface acoustic wave that propagates along an axis on which a reflective grating is disposed to reflect portions of the surface acoustic wave along plural parallel paths of differing lengths. The reflective gratings associated with the respective transducers are disposed on perpendicular axes so as to provide a grid pattern to enable coordinates of a touch on the plate to be determined. Acoustic touch position sensors of this type are shown in U.S. Pat. Nos. 4,642,423, 4,644,100, 4,645,870, 4,700,176, 4,746,914 and 4,791,416.
Acoustic touch position sensors utilizing surface acoustic waves as taught by the above-mentioned patents have a number of problems which are more readily understood when the nature of the surface acoustic wave used in these sensors is considered. Surface acoustic waves are shown in FIGS. 1A-D propagating in the X direction. Surface acoustic waves have an X component and a Z component such that the particles of a surface acoustic wave move elliptically in the X, Z plane. Although surface acoustic waves have a Z component, the disturbance of particles in the plate created by a surface acoustic wave decays rapidly in the -Z direction so that the wave energy is essentially confined to the surface of the plate.
More precisely, waves in a uniform, non-piezoelectric medium of finite thickness that are confined to a single surface are termed quasi-Rayleigh waves, since true Rayleigh waves exist only in an infinitely thick propagating medium. A Rayleigh/quasi-Rayleigh wave is shown more particularly in FIG. 1D. In order to provide such a wave, the thickness of the touch plate must be at least three to four times the wavelength of the wave imparted into the substrate wherein the length of the touch plate is also limited. If the thickness of the touch plate is for example two wavelengths or less, Lamb waves will be generated in the touch plate instead of Rayleigh waves. Lamb waves are dispersive waves, varying in phase and group velocities. A touch plate in accordance with the teachings of the above-mentioned patents would not work in such a thin plate because Rayleigh or quasi-Rayleigh waves cannot exist therein. However, for a panel having a thickness that is at least three to four times the wavelength of the wave propagating therein, nearer the source of the wave, i.e. the transducer, the symmetric and anti-symmetric Lamb waves appear to be in phase. As seen in FIG. 1D, the symmetrical and anti-symmetrical Lamb waves 13 and 14 are not confined to a single surface of the touch plate but extend through the plate to the opposite surface thereof. When in phase, however, the symmetric and anti-symmetric Lamb waves add to produce a quasi-Rayleigh wave as can be seen from a comparison of FIGS. 1E and 1F to FIG. 1D. As the Lamb waves travel farther from the transducer, due to the differing phases and velocities of the Lamb waves, there is a complete transference of wave energy from the top surface of the touch plate on which the transducer is mounted, to the bottom surface of the touch plate. This transference of energy between top and bottom surfaces occurs at regularly spaced intervals making a touch plate having a dimension large enough for this transference of energy to occur unsuitable for a touch position sensor.
From the above it is seen that touch position sensors as shown in the above-mentioned patents utilizing surface acoustic waves and more particularly quasi-Rayleigh waves, as is necessary for the sensors to operate, are limited to relatively thick panels, i.e. panels having a thickness of three to four times the wavelength of the surface acoustic wave propagating therein. Although the wavelength of the propagating wave may be reduced by reducing the frequency of the drive signal applied to the transducer, as the wavelength of the wave is reduced, transference of energy between the top and bottom surfaces of the touch plate occurs closer to the transducer so as to limit the size of the touch plate.
Further, because surface acoustic waves are confined to the surface of the touch plate, contaminants or other materials abutting the plate may create shadows or blind spots extending along the axes of the plate that intersect the contaminant or abutting material. The blind spots or shadows are created by a total or near total absorption of the wave energy by the contaminant or abutting material so that the touch position sensor cannot detect a touch if one of its coordinates is on a blinded axis. Substantial losses in wave energy over time as a result of air damping of the surface acoustic wave is also significant since surface acoustic waves are confined to the surface of the touch plate. The energy losses due to air damping further limit the size of the touch plate.
Although acoustic waves other than surface acoustic wave can propagate in a solid such waves including Lamb waves and shear waves, heretofore these other acoustic waves were thought to be unsuitable for a touch position sensor. Lamb waves were thought unsuitable because they are dispersive, varying in phase and velocity, so as to interfere with one another. Shear waves were thought unsuitable because a touch on a plate in which shear waves are propagating absorbs only a small percentage of the total shear wave energy whereas a touch on a plate in which a surface acoustic wave is propagating absorbs a much greater percentage of the surface acoustic wave energy. More particularly, the percentage of total energy absorbed by a given touch is ten times greater for a surface acoustic wave than it is for a shear wave. Since shear waves are not nearly as responsive to touch as surface acoustic waves, shear waves were not thought practical for a touch position sensor.